Changing the separation distance of y-axis labels on a QwtPlot

How does one change the separation distance of y-axis labels on a QwtPlot? I've mucked up the barchart.cpp example. I created a simple TextScaleDraw : public QwtScaleDraw that I pass to the QwtPlot with setAxisScaleDraw. I see my labels fine, but there is a lot of wasted vertical space between the labels (about 4 times the height of the labels).

Upon further evaluation, the labels are put at the major tick marks, so the real question is how to change the physical spacing of the major tick marks. I'm not displaying the minor tick marks, but it appears that physical space is being allocated for them. I need to eliminate this physical spacing.